Boulder Retaining Wall and Related Drainage Issues

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Boulder retaining wall

The above picture demonstrates how a boulder retaining wall has moved along side the foundation of a home since its initial construction.  Boulder retaining walls are a common landscaping and erosion control method used in many homes.  If a foundation wall is over 48 inches, it needs to be designed by an engineer and requires plans for ordinance and structural review by the local governing body.

There are many elements that can encourage boulder wall movement, but here are

the three main causes:

1.) Often, the wall has been built on unsuitable soils that lacked compaction and ultimately could not support the weight of the wall. Contributing further to this problem, is the added pressure from the soil and related area behind the wall.

2.) Secondly, sometimes retaining walls are built with improper drainage behind it, including no installation of drain tile, drainage material behind the rocks and/or fabric to hold that material in place.

3.) Thirdly, the poor drainage of rain water behind the wall causes erosion to take place and further soil settlement under the boulder wall itself.  This water often comes from a roof or rain gutter that empties behind the wall.

In this case, as the picture depicts, some of those elements were present. This wall had movement subsequent to its initial construction.  This resulted in damage to the outside parge coat on the foundation wall of the home and a leak developed in the basement adjacent to this boulder wall.

Here are some of the things to consider when constructing a boulder retaining wall:

  1. How high is the rock wall going to be? Will it need to be engineered?
  2. Location of the rock wall, asking is it on stable slopes, consisting of firm, undisturbed soil?
  3. The ground surface above the Boulder Wall. How much water will come through this area?
  4. The proper angle of the rockery face of the Boulder Retaining Wall.
  5. The proper rock size should be used to construct the wall.
  6. Rock placement including the rocks to be embedded into the soil at the base to provide maximum stability.
  7. The drainage must be provided behind the wall to assure that water will not erode the soil behind or underneath he boulders. This includes the use of a drain tile system, erosion fabric, and drainage material such as gravel behind the boulder wall itself.

As a homeowner, before taking on construction of a boulder wall, you may want to check with experts in this area.

 

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